A typical method for forming medical device balloons includes molding the balloon from a hollow parison, for instance an extruded tubular parison. The mold form for such processes is adapted to receive the parison of thermoplastic material and has a cavity into or through which the parison extends. Heat is applied to the mold, for instance by immersion in a heated fluid, to soften the thermoplastic material. Concurrently, or in some sequence of heating and pressurization, the parison is pressurized to radially expand the softened thermoplastic material until it contacts and conforms to the shape of the cavity. This typical method may be practiced in a variety of known ways, for instance with or without an axial stretch step, which may be performed concurrent with or prior to radial expansion and at temperature above or below a glass transition temperature. A heat setting step or an annealing step may be performed after the balloon has been molded.
In mass production of medical device balloons, some processes produce substantial rejection rates. Parison shaping techniques, including multi-step balloon blowing processes going beyond simple axial stretching and radial expansion of straight tubes, are known. Examples of such techniques include U.S. Pat. No. 4,963,313, Noddin et al; U.S. Pat. No. 5,017,325, Jackowski et al; U.S. Pat. No. 5,334,146, Ozasa; U.S. Pat. No. 5,500,180, Anderson et al, U.S. Pat. No. 5,525,388, Wand et al; U.S. Pat. No. 5,714,110, Wang et al; U.S. Pat. No. 5,826,588, Forman; U.S. Pat. No. 5,948,345, Patel; U.S. Pat. No. 6,193,738, Tomaschko et al; and U.S. Pat. No. 6,458,313, Hudgins et al. At least some of these techniques, however, tend to increase balloon rejection rates when implemented in mass production.
When molding balloons utilizing certain polymer systems, for instance polyamide polymers such as nylon 12; polyester polymers such as polyethylene terephthalate, polyethylene naphthalate and polybutylene terephthalate; polyurethane polymers such as Pellethane® 2363-75D, multi-step blowing processes have been recommended. In such processes the parison is successively expanded in two or more radial expansion steps, each of which may be accompanied by a prior or concurrent axial stretching step.
In U.S. Pat. No. 5,348,538, Wang et al, there is described a tower molding apparatus for catheter balloons that immerses a parison/mold form in heated liquid media. A variant of this type of apparatus is also described in U.S. Pat. No. 5,714,110, Wang et al. In such apparatus parison tubing is threaded through a multi-part mold, the tubing is connected to an associated pressurized fluid source and the mold mounted in the apparatus in a largely manual process. Multi-step expansion has not previously been employed commercially in such an apparatus.